In a communication system, a device (typically a mobile device) termed user equipment (UE) communicates wirelessly with a core network via a radio access network. Communications between the UE and the radio access network are effected in accordance with a multi-layered communications protocol.
Upon detecting that a call is to be established, the UE implements a procedure to establish a Radio Resource Control (RRC) connection between the UE and the radio access network. This known procedure is described in the 3GPP TS 25.331 specification. If there is a logical connection between the UE and the radio access network, then the UE is said to be in a connected mode. Once in the connected mode, the UE transmits an indication that a mobile originating call is required to the radio access network.
In response to receiving the indication from the UE that a mobile originating call is required, the radio access network initiates a Security Mode Command procedure in order to establish a secure channel between the UE and radio access network. This known security procedure is described in detail in the 3GPP TS 25.331 specification.
The security consists of two aspects, optional ciphering and mandatory integrity protection. Ciphering provides encryption according to a ciphering configuration to ensure that all signalling and data messages transmitted between the UE and the radio access network are ciphered over the air interface to provide data confidentiality. Integrity protection provides protection against message manipulation between the UE and the radio access network. That is, integrity protection prevents third parties from sending unauthorised signaling messages between the UE and the radio access network. Typically, both ciphering and integrity protection are enabled during a call.
As part of the Security Mode Command procedure the radio access network sends a Security Mode Command using the Radio Resource Control (RRC) protocol to the UE with an indication of a new integrity protection configuration and new cipher configuration. In response to receiving the Security Mode Command, the UE sends a RRC Security Mode Command Complete message to the radio access network. The radio access network completes the security mode command procedure successfully when the Security Mode Command Complete message is received.
After sending the Security Mode Command Complete message to the radio access network, the UE transmits a call setup message to the radio access network. If the call setup message is correctly received by the radio access network after the Security Mode Command procedure has successfully completed (i.e. a security configuration has been successfully established between the UE and the radio access network) then the radio access network will respond to the call setup message by transmitting a call proceeding message to the UE 102 and proceed with the known mobile originating call establishment procedure which is described in detail in the 3GPP TS 24.008 specification.